In printing environments, the transport of paper, or other sheets upon which text and images are rendered, is one of many important components in the overall quality of the printed sheet. In this regard, accuracy and precision of registration of the sheets to the text and images printed thereon contribute to the print quality. If the sheets are not transported in an acceptable manner, then the registration process could be adversely impacted.
In high speed, high end printing environments, a technique of agile registration has developed. Agile registration relates to registration techniques that involve high speed, adaptive, closed loop processes.
More particularly, with reference to FIG. 1, a document processing device 10 is illustrated. The device 10 includes a controller 12 that controls a variety of functions of the device including the paper path. In this regard, the paper path includes stationary nips A and B which impart x-direction velocity vectors VA and VB on a sheet 14. The average (VA+VB)/2 provides an x-direction (process direction) motion to the sheet 14. The difference (VA−VB) provides a rotation of the sheet 14. The sheet 14 is to be delivered to a device downstream. This device can be a photoreceptor or a drum (where it can receive an image) or any other appropriate device, inclusive of another set of nips.
In known processes, before the sheet 14 enters the nips A and B, the velocities VA and VB are typically set equal to the paper velocity of the upstream paper path V0. This should assure correct hand-off of the sheet from the upstream path to the paper registration device.
In this regard, agile registration commences shortly after the paper arrival as detected by sensors LEA and LEB. The sensors report the time-of-arrival t0 and the process position x0 and angle β0 of the sheet. The side edge, or lateral, sensor reports the lateral position y0. In many cases, the lead-edge-center or lead-edge-side is considered the point that is being registered. Simple geometric calculation will yield values for the initial conditions of the registration point from sensor measurements.
Typically, delivery strategies calculate velocity profiles VA(t) and VB(t) to deliver the sheet 14 from these initial conditions to an end condition. The velocity profiles VA(t) and VB(t) must be calculated to deliver the sheet to position xf, yf, βf at a time tf with a velocity vf. As noted above, the velocity vf usually matches the velocity of the downstream device. However, in actual implementation, there are many factors that detract from these expectations.
In this regard, agile registration processes using polynomial profiles have been used. However, while polynomial agile registration typically exhibits accurate registration results, a large tail wag is generated. Triangular profiles have also been used. These profiles typically result in a small tail wag, but have less accurate results. Use of trapezoidal profiles has advantages, but typically leads to unpredictable nip forces.
It is desired that velocity profiles be calculated more accurately than is presently known to obtain precise delivery of sheets at various points in the paper path to achieve desired paper registration.